It has been known to open or expand, or to block, respectively, vessels and other cavities in the human body by use of inflatable catheters. To this end dilation catheters are used which have an expansion or dilation element at their end. When positioning the catheter, the dilation element is positioned at the location to be dilated and then pumped up. It is a disadvantage during pumping-up that the vessel or other body cavity in question be blocked by the dilation element because the supply of blood to the upstream or next in line organs, for example, will be interrupted for the duration of the treatment.
An object of the invention is to provide a dilation device of the above-mentioned type which permits alternating expansion and contraction of the dilation element in rhythm with the heart pulse or frequency, to free the vessel or the body cavity in question so that fluid can circulate between short-term dilation phases.
In order to satisfy this object there is provided, according to the invention, a pressure source means which supplies a pulsating pressure, the lower pressure value of which is synchronized, as an operating pressure, with the expansion characteristics of the dilation element so that it lies near but below the steep volume increase, in the area of the steep pressure increase at about a constant volume, and the upper pressure value of which lies in the area of the steep volume increase, whereby a change of the pressure p causes a defined volume change v, and in which the dilation element is so designed that its volume above the operating pressure can follow pressure pulses with a pulsing frequency of 0.5 to 3 Hz.
The invention is based on the idea that the expansion of a dilation element forming a component of a catheter normally has a delay or phase shift towards the pressure pulses of the pressure source as compared to the pressure charge by means of the pressure source, if the pressure of the source is reduced to about zero each time between two pressure pulses. This can be attributed to the flow resistance of the catheter and to the pressure-volume characteristics of a balloon-type dilation element. When the dilation element is pumped-up from a pressure-free condition, the pressure in the dilation element first increases without causing a substantial increase in volume. Only after a certain pressure, which may be called a "knee" or inflection point, does a substantial volume gradient result, that is, the volume of the dilation element greatly increases with increasing pressure. By means of the invention, the operating pressure is set to be closely beneath the inflection point of the pressure-volume curve and so that as well, a certain minimum or pre-pressure is maintained in the dilation element between pressure pulses. The increase of fluid pressure which is necessary to increase the volume of the dilation element is relatively small starting from this minimum or pre-pressure. In this way it is possible, with small pressure changes on the liquid in the catheter, to cause large volume changes in the dilation element so that the dilation element can be pumped-up or expanded in rhythm with the heart pulse or frequency and then contracted. As a result, it is not necessary to carry out a cycle consisting of pumping-up and contraction of the dilation element with each pulse beat but, instead, the dilation element can remain pumped-up and be contracted over one or several pulse beats. It is only important that the process of pumping-up and contraction takes place relatively quickly and with as low an inertia as possible.
A beneficial feature of the invention includes the provision of a catheter comprising a tube which can extend from a fluid pressure source connection to the dilation element, and with the tube having a radial expandability less than that of the dilation element. It is preferred that the diameter of the tube in a zone adjacent the dilation element be relatively small, and that the remainder of the tube, leading to the pressure source, be enlarged. This enlargement of the tube diameter, which reduces flow resistance through the catheter, together with the limited expandability of the tube increases the upper limiting frequency for transmission of pressure pulses to the dilation element. Preferably, the upper limiting frequency for the transmission of pressure pulses from the pressure source to the dilation element lies at about 100 Hz, thereby permitting higher frequency pressure fluctuations to be superimposed on the pressure pulses, which, for example, have a triangular, sawtooth-type or sinusoidal time sequence. Due to these types of fluctuations, which are superimposed on a pressure pulse, the vessel dilation is improved.
In order to make a pressure measurement possible during advancement of the catheter to the dilation location, a particularly useful catheter includes a tube with a pressure measuring channel, the front end of which extends past or beyond the dilation element. The extending portion is soft so that injuries to body cavities and vessels are avoided.
Following hereinafter is an example describing the invention in detail with reference to the attached drawings.